Recent increase of mobile data usage and emergence of new applications such as gaming, mobile TV and streaming content have motivated the 3G Generation Partnership Project (3GPP) to work on the Long-Term Evolution (LTE) in order to ensure 3GPP's competitive edge over other, competitive cellular technologies.
LTE has been set aggressive performance requirements which rely on physical layer technologies, such as Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) systems to achieve these targets. Some main objectives of LTE are to minimize the system and User Equipment (UE) complexities, to allow flexible spectrum deployment in existing or new frequency spectrum, and to enable co-existence with other 3GPP Radio Access technologies (RATs).
In the LTE concept defined in the ongoing 3GPP work on standardization, the downlink will support fast channel dependent scheduling in both the time and frequency domains. A conventional downlink scheduling concept according to the prior art, may be described with stages 1:1-1:4, as illustrated in FIG. 1. A base station 100, which is referred to as an enhanced NodeB, or eNodeB in LTE, communicating with a UE 101, transmits reference signals to UE 101 in a first stage 1:1. The reference signals can be used by UE 101 to determine the present downlink channel quality. After having determined the downlink channel quality on the basis of the received reference signals, UE 101 sends one or more channel state feedback reports, which in this context typically are represented by Channel Quality Indication (CQI) reports, back to eNodeB 100 in a second stage 1:2. In eNodeB 100, the content of the one or more CQI reports can be retrieved and used by a scheduler (not shown), to perform resource allocation. UE 101 is informed of the resource allocation in a next stage 1:3, which is followed by transmission of downlink data over the allocated resource, as indicated with a final stage 1:4.
More on this issue can be found in “3G Evolution: HSPA and LTE for Mobile Broadband” E. Dahlman, S. Parkvall, J. Sköld, P. Beming, Academic Press, 2007.
In one embodiment proposed for the LTE, UEs will be capable of transmitting different types of CQI reports, such as full CQI reports, partial CQI reports, and differential CQI reports. In this context, full CQI reports are defined to cover the whole downlink transmission bandwidth scheduled for a UE. Different full CQI reports may, however, have different frequency resolution and may also be filtered and processed in different ways. In addition, different full CQI reports may be encoded in a variety of alternative ways. Partial CQI reports on the other hand cover only a part of the downlink transmission bandwidth. The covered part of a partial CQI report may be a set of contiguous, or a set of distributed resource blocks. Differential CQI reports may contain an encoded version of the update vector relative to a previous CQI report.
Furthermore, for CQI reports used together with different antenna configurations, such as SISO (Single-In Single-Out), MISO (Multiple-In Single-Out), SIMO (Single-In Multiple-Out), or MIMO (Multiple-In Multiple-Out), transmission could also be different. For MIMO, a CQI report may include information, such as e.g. transmission rank and/or pre-coding weights and/or other feedback parameters to be used by the eNodeB multiple antenna transmission scheme.
In one proposal for LTE presented in 3GPP, the UE may have a set of rules that specifies the conditions for CQI reports to be transmitted. According to this proposal, each CQI transmission trigger is associated with a specific type of CQI report in such a way that when a triggering criteria is true, the UE transmits a CQI report of an associated type. This procedure is similar to how compressed mode is parameterized in WCDMA. For WCDMA compressed mode, each UE is provided with a transmission gap pattern set (TGPS) consisting of transmission gap patterns (TGP), each defining a transmission gap of a configurable length that is used for a specific measurement purpose. CQI reports may be specified in a similar way, wherein each UE has a CQI reporting trigger set (CRTS), consisting of one or more CQI reporting triggers (CRT) that specify when a specific type of CQI report shall be transmitted.
FIG. 2 illustrates a table of a CQI trigger configuration for a UE, according to the prior art described above. The table comprises a plurality of CQI reporting triggers, CRT 1-n, configured for the UE. Each CRT is associated with one of the CQI report types, CQI A-X. When for example the trigger criteria specified by CRT 1 is true, a report type defined by CQI A will be transmitted from the UE to an eNodeB, as indicated in the table.
A CRT is typically expressed in terms of a logical expression which may involve one of, or a combination of, timers, events, and conditions, consisting of logical statements such as AND, OR, NOT, WHEN, and/or IF. A simple periodic CQI reporting trigger may consist of just a periodic timer and a rule that a certain CQI report shall be transmitted every time the timer expires. In another exemplified scenario, a simple event based CQI reporting trigger may configured to state that a certain type of CQI report shall be transmitted every time the triggering event, such as e.g. a handover event, occurs. A condition that could be included in the decision to transmit a certain CQI report or not, is e.g. if the downlink activity is above a specified threshold.
CQI reports may also be transmitted in different ways. A CQI report could be transmitted on a dedicated control channel resource, or on a scheduled resource provided on a shared channel. CQI reports may occur at known time instances and use a format known to the eNodeB, or the occurrence and format may be more dynamic. In the latter case the MAC header typically needs to include information about how the CQI report was transmitted, or else the eNodeB may have to perform blind detection on the CQI transmission format.
What types of CQI reports a UE shall use, and what criteria that will trigger them, are typically set-up by higher layer signaling, e.g. RRC signaling. In addition to configuring rules, defining when and how CQI reports are to be transmitted, the eNodeB also have the option to explicitly request for CQI reports on demand, typically by using RRC signaling.
The LTE uplink is based on single-carrier modulation and uses frequency, time and code division multiple access principles (FDMA, TDMA and CDMA). The LTE uplink consists of physical uplink control channels and data channels that are orthogonally frequency multiplexed. The single-carrier property of the LTE uplink makes it impossible for a UE to transmit on a physical control channel and a physical data channel in the same transmission-time-interval (TTI). Hence, if a UE is transmitting data on a physical data channel, the control information that must be sent in the same TTI must also be sent on the physical data channel. The UE will use the physical control channel to transmit control signaling only in the case when the UE has no data transmission, and, hence is not using the physical data channel.
There are at least three types of control signaling that may be sent in-band on the physical data channel in case the UE has uplink data to transmit, namely Hybrid ARQ (HARQ) ACK/NACK feedback for downlink data transmissions, scheduling requests and CQI reports.
The current assumption in 3GPP regarding the HARQ feedback and the scheduling request is that the HARQ will consist of one bit per MIMO stream, while the scheduling request might consist of just a single bit, indicating if a UE has data it wants to transmit or not.
The CQI reports on the other hand can be significantly larger. The amount of bits that can be spent on the CQI reporting may depend on a number of different criteria, such as: downlink transmission mode, e.g. SISO or MIMO; type of downlink traffic, e.g. VoIP or Web; downlink radio characteristics, e.g. coherence time and/or coherence bandwidth; current uplink load and/or current downlink activity. Furthermore, while the HARQ feedback and the scheduling request signaling are vital for the communication protocols to work at all, the CQI reports can be seen more as performance enhancing feature for the downlink.
The more uplink resources that are spent on CQI reports, the better link adaptation and scheduling decisions can be made, and the better the performance of the downlink may be achieved. As for signaling in general, there is, however, a trade-off between the amount of resources that are used for signaling and the amount of resources available for transmission of user plane data traffic. In current state-of-the-art it is known that it is beneficial to adapt the CQI reporting scheme to the conditions listed above.
A drawback with prior art CQI reporting mechanisms is, however, the lack of flexibility as to the use of available resources.
In order to fully support all possible CQI feedback schemes in all possible scenarios one would need to allocate an unreasonable amount of physical resources for uplink physical control signaling.
Even with a limited number of schemes applied, new feedback schemes are difficult to introduce, especially if they require that the uplink physical control channels need to be re-designed.